It is required to further increase a recording density of magnetic recording media with the shift of a writing-reading system from an analog system to a digital system. In particular, when video tapes and backup tapes of computers, which face severe competition with hard discs or optical discs, cannot satisfy the above requirement, the continuance of the products may be endangered.
To satisfy the requirement to the increase the recording density, magnetic recording media comprising a thin film of a magnetic layer are proposed. However, so-called coating type magnetic recording media, which are produced by applying a magnetic paint containing a magnetic powder dispersed in a binder on a non-magnetic support, are superior to the thin metal film type ones in view of the productivity, and practical reliability such as corrosion resistance. Roughly speaking, the electromagnetic conversion characteristic of the coating type magnetic recording media has been improved by the improvement of magnetic powders and the improvement of production methods.
In connection with the improvement of the magnetic powders, the magnetic properties are year-by-year improved in conjunction with the miniaturization of the particle size to cope with the short-wavelength recording. Formerly, magnetic powders such as ferromagnetic iron oxide powder, cobalt-modified ferromagnetic iron oxide powder and chromium oxide powder, which are used for audio tapes or domestic video tapes, are mainly used also for high density recording magnetic recording media, but recently acicular metal magnetic powders having a particle size of about 0.1 μm is proposed for the high density recording magnetic recording media.
To prevent the decrease of output due to the demagnetization in the short wavelength recording, a coercive force has been increased year-by-year, and the alloy of iron-cobalt achieved a coercive force of about 198.9 kA/m (see U.S. Pat. No. 5,252,380, JP-A-5-234064, JP-A-6-25702, JP-A-6-139553, etc.)
In connection with the improvement of the production methods of the magnetic recording media, the use of binders having various functional groups, the improvement of the dispersing technique of the above magnetic powders, and the improvement of the calendering method after the application process can remarkably increase the surface smoothness of the magnetic layers, and thus greatly contribute to the increase of the output in the short wavelength range (see U.S. Pat. Nos. 4,324,177, 4,952,444, JP-A-4-19815, etc.)
However, since the recording wavelength is shortened with the recent increase of the recording density, the influences of self-demagnetization loss in the course of writing and reading and thickness loss due to the thickness of the magnetic layer, which have not caused any problem, increase, and thus sufficient dissolution may not be attained. Such problems cannot be solved by the above-described improvement of the magnetic properties of the magnetic powders or the increase of the surface properties achieved by the production methods of the media. Thus, it is proposed to decrease the thickness of the magnetic layer.
In general, the effective thickness of the magnetic layer is about one third (⅓) of the shortest recording wavelength used in a system. For example, with the shortest recording wavelength of 1.0 μm, the thickness of the magnetic layer should be about 0.3 μm. Furthermore, with the miniaturization of a cassette, the whole thickness of the magnetic recording medium should be decreased to increase a recording capacity per unit volume. Consequently, the thickness of the magnetic layer should be decreased. In addition, to increase the recording density, the area of a writing magnetic flux, which is generated with a magnetic head, should be decreased, and thus the magnetic head is miniaturized. Therefore, the amount of the generated magnetic flux decreases. Accordingly, the magnetic layer should be made thin to cause complete reversal of magnetization with the minute magnetic flux.
When the thickness of the magnetic layer is decreased, the surface roughness of the non-magnetic support has some influence on the surface of the magnetic layer and thus the surface properties of the magnetic layer tend to deteriorate. Furthermore, when the thickness of a single magnetic layer is decreased, it may be contemplated to decrease the solid concentration of a magnetic paint or to decrease the amount of the magnetic paint applied. However, these methods cannot prevent defects formed in the course of application, or achieve the increase of the filling of the magnetic powder. Therefore, the strength of the coated film may deteriorate. Accordingly, to decrease the thickness of the magnetic layer by the improvement of the production methods of the magnetic recording media, a so-called simultaneous multiple layer coating method is proposed, which comprises providing an undercoat layer between a non-magnetic support and a magnetic layer, and applying a magnetic paint of the upper magnetic layer while the undercoat layer is still wet (see U.S. Pat. Nos. 4,863,793, 4,963,433, 5,645,917, 5,380,905, 5,496,607, etc.)
With such improvements of the coating methods, it becomes possible to thinly coat a magnetic layer having a thickness of about 1.0 μm, and such thin film-coating methods and the above-described improvement of the magnetic powders can solve the various problems such as the decrease of the output caused by the demagnetization, which is the essential problem of longitudinal recording.
However, in these days, the improvements of the magnetic powders and the production methods of the magnetic recording media reach the limits. In particular, in the case of the improvement of the magnetic powders, insofar as the acicular magnetic powder is used, the practical lower limit of the particle size is about 0.1 μm, because when the particle size is less than about 0.1 μm, a specific surface area of the particle increases greatly, and thus not only the saturation magnetization decreases but also the dispersion of the magnetic powder in the binder becomes very difficult.
In connection with the coercive force, signals can be recorded on magnetic recording media having a very high coercive force because of the technical innovation of the magnetic heads. In particular, in the case of the longitudinal recording system, it is desirable to increase the coercive force to as high as possible to prevent the deterioration of the output due to the writing and reading demagnetization, insofar as the recorded signals can be erased with the magnetic head. Accordingly, the realistic and most effective method to increase the recording density of the magnetic recording media is to increase the coercive force of the media.
It is effective to further decrease the thickness of the magnetic layer to suppress the influence of the decrease of the output caused by the writing and reading demagnetization, which is the essential problem of the longitudinal recording. However, the thickness of the magnetic layer will reach the limit, insofar as the above-described acicular magnetic powder having a particle size of about 0.1 μm is used. The reason is as follows: the acicular particles are aligned in the plane direction of the magnetic recording medium on the average by longitudinal orientation, but some particles may be aligned in the direction perpendicular to the plane of the medium since the orientation of the particles has distribution. When such particles are contained, they deteriorate the surface smoothness of the medium and may increase noise. Such problems become more serious as the thickness of the magnetic layer decreases.
When the magnetic layer is made thin, it is necessary to dilute the magnetic paint with a large amount of an organic solvent. However, the conventional miniaturized acicular magnetic powder particles tend to cause the agglomeration of the magnetic paint. In addition, since a large amount of the organic solvent is evaporated when the applied magnetic paint is dried, the orientation of the magnetic powder particles is tend to be disturbed. Thus, in the case of tape-form media which are longitudinally recorded, the desired electromagnetic conversion may not be attained because of the deterioration of the orientation and the surface properties, even if the magnetic layer is made thin. Thus, it is very difficult to produce coating type magnetic recording media having the further decreased thickness of the magnetic layer, insofar as the conventional acicular magnetic powder is used, although it is known that the decrease of the thickness of the magnetic layer is effective to increase the recording characteristics of the media in the case of longitudinal recording.
Among the already proposed magnetic powders, the barium ferrite magnetic powders having platelet particle shapes, and comprising very fine magnetic particles with a particle size of 50 nm are known (see JP-B-60-50323, JP-B-6-18062, etc.) The shapes and particle sizes of the barium ferrite magnetic powders are more suitable for the production of the thin-layer coating type magnetic recording media than the acicular magnetic powders. However, since the barium ferrite magnetic powder is an oxide, its saturation magnetization is at most about 7.5 μWb/g, and thus it is theoretically impossible to achieve a saturation magnetization of 12.6 μWb/g or more, which is the level of the saturation magnetization of acicular metal or alloy magnetic powders. Therefore, when the barium ferrite magnetic powder is used, the high output cannot be attained since the saturation magnetization is low, although the coating type magnetic recording media comprising a thin magnetic layer may be produced. Thus, the barium ferrite magnetic powders are not suitable for the high recording density magnetic recording media. For the above reason, the above-described acicular magnetic powders has been dominantly used as the magnetic powders for the high recording density magnetic recording media.
As explained above, it is a very important problem to reduce the particle size of a magnetic powder while maintaining the coercive force and saturation magnetization at a as high level as possible to reduce thickness of the magnetic layer, which is an effective measure to increase the recording density of the magnetic recording media. To solve such a problem, firstly, the magnetic characteristics of the conventional magnetic powders are discussed. In the case of the currently used acicular magnetic powders, the increase of the coercive force has a limit theoretically, since its coercive force is based on the shape anisotropy of the acicular particles. That is, the magnetic anisotropy based on the shape anisotropy is expressed by 2πIs wherein Is is a saturation magnetization, and thus proportional to the saturation magnetization. Thus, the coercive force increases as the saturation magnetization increases in the case of the acicular magnetic powders the coercive force of which is based on the shape anisotropy.
The saturation magnetization of a magnetic metal or alloy, for example, an Fe—Co alloy reaches the maximum near a Fe/Co ratio of 70/30, as is well known from the Slater-Pauling's curve. Therefore, the coercive force also reaches the maximum at the above composition of the alloy. The acicular magnetic powder of such a Fe—Co alloy having a Fe/Co ratio of about 70/30 is already practically used.
The magnitude of the magnetic anisotropy based on the shape anisotropy is expressed by 2πIs as explained above. The factor is about 2π when the acicular ratio (particle length/particle diameter) of the magnetic powder is about 5 or more, but the factor quickly decreases when the acicular ratio is less than about 5. Finally the anisotropy disappears, when the particle becomes a sphere. That is, insofar as magnetic materials of metal iron or Fe—Co alloys are used as the magnetic powders, the shape of the magnetic powder particles should be in the acicular form (needle form) from the theoretical viewpoint.